The invention relates to a process for producing a microporous hydrophobic inorganic membrane comprising hydrolysing a metal alkoxide in the presence of a hydrocarbyl metal alkoxide in an organic solvent to produce a metal (hydr)oxide sol, and precipitating metal (hydr)oxide from said sol and drying and calcining the precipitate.
Such a process for producing hybrid membranes has been described by Dirxc3xa8 et al, J. Mater. Chem., 1997, 7, 67-73 and 919-922. These authors report the production of unsupported membranes by hydrolysing a mixture of tetraethoxysilane and methyl-triethoxysilane in 70/30, 50/50 and 30/70 ratios at room temperature and at pH 1.5. The resulting sol was allowed to form a gel having a thickness of 10-40 xcexcm and dried. Gas permeation for helium, nitrogen and argon was studied, and a selective permeation of helium to nitrogen was found, depending on the methyl content of the gel membrane. However, the membrane has relatively large pores, the majority of pore diameters being above 2 nm.
An improved method has now been found which results in hydrophobic membranes, both supported and unsupported, having pores sizes in the subnanometer range, and having permeances for small molecules such as H2, CO2, N2, O2 and CH4 and good separation factors for these gasses with respect to larger alkanes such as propane and butane, and for compounds such as SF6. The process of the invention is characterised by a controlled hydrolysis in that the hydrolysis rate of the hydrocarbyl metal alkoxide is taken into account in the rate of sol formation of the metal oxide, by adding a substantial part of the hydrocarbyl metal alkoxide after part of the metal alkoxide has been allowed to hydrolyse. The controlled hydrolysis is defined in the appending claims.
The invention also relates to novel hydrophobic membranes having improved characteristics over the prior art membranes, especially an average pore size (diameter) and/or a majority of pore sizes below 2 nm, or even below 1 nm, and a hydrophobicity index (HI) for octane/water of at least 2, especially at least 2.5 or even 3 or higher. The hydrophobicity index is defined as the ratio of loading of the membrane with hydrophobic substance (octane) to the loading with water as described by Klein and Maier (Angew. Chem. Int. Ed. Engl. 35, (1996) 2230-2233; and by Weitkamp et al (Proc. 9th Intern. Zeolite Conf. Montreal 1992, Von Balmoos et al Eds. (1993) 79-87). The membranes show high permeances for small molecules, and especially allow separation of small hydrophobic molecules from water or from other hydrophobic molecules, also in the presence of water (humid process streams). Larger molecules such as butane and methyl tert-butyl ether have permeances which are more than 20 times lower than those of the small molecules such as H2, methane, methanol etc.
According to the process of the invention the larger part of the hydrocarbyl metal alkoxide is added after at least 20% of hydrolysis of the metal alkoxide has occurred. The percentage of hydrolysis occurred can roughly be considered equivalent with the percentage of the xe2x80x9cnormalxe2x80x9d hydrolysis period that has lapsed, but more precisely it is the percentage of the total theoretical hydrolysis of metal alkoxide to metal (hydr)oxide. The total amount of hydrocarbyl metal alkoxide used means the total amount added to the hydrolysis system for the purpose of being incorporated into the membrane after hydrolysis. The larger part introduced after at least 20% of hydrolysis is at least 75% of the total amount, but especially essentially the total amount. It is further preferred that a substantial part, especially at least 50%, of the hydrocarbyl metal alkoxide is added only at a later stage, i.e. after at least 50% or even at least 70 or 80% hydrolysis of the metal alkoxide.
The terms metal oxide and metal (hydr)oxide are used interchangeably and denote any oxide, hydrated oxide, hydroxide or mixed oxide/hydroxide of any trivalent or higher-valent element from period 3 and higher periods and groups 3 and higher groups of the periodic table of elements, including e.g. Al, Si, Sc, Ti, V, Ge, Sn, Hf, Ce, as well as combinations thereof such as Si/Ti and Si/Zr.
In the term metal alkoxide as used herein, the metal is defined as above, whereas an alkoxide is understood to comprise the residue obtained by deprotonation of any organic molecule containing a hydroxylic group and an alkyl group; the hydroxyl group may directly be attached to the alkyl group, such as in the methanol, ethanol, propanol, isopropanol, butanol and the like, but also through a carbonyl group as in carboxylic acids (acetic acid, propionic acid and the like); the hydroxyl group may also be an enol tautomer of a ketone, especially a xcex2-diketone or xcex2-ketoester, such as acetyl-acetone (=4-hydroxy-3-propene-2-one). Examples of metal alkoxides include tributoxyaluminium, tetraethoxysilane (TEOS), tetra-isopropoxysilane, tetrabutoxytitanium, tripropoxytitanium acetoacetonate, tributoxytitanium acetate, tetrabutoxyzirconium, tripropoxyzirconium, tetraethoxytin.
A hydrocarbyl metal alkoxide as used herein denotes a compound of a tri- or higher-valent metal as described above with at least one alkoxide group as defined above and at least one hydrocarbyl group which is bound to the metal with a bond which, under normal conditions (temperature below 100xc2x0 C., pH between 1 and 13), is non-hydrolysable. Such a hydrocarbyl group can be any organic radical containing from 1 to 8 carbon atoms and the corresponding number of hydrogen atoms, such as methyl, ethyl, butyl, isooctyl, phenyl and benzyl. Small alkyl groups, i.e. with 4 or less carbon atoms, especially methyl and ethyl are preferred. Examples include methyl-triethoxysilane (MTES), phenyl-trimethoxysilane, diethyl-dipropoxytitanium, methyl-dibutoxyzirconium acetate, and the like.
The hydrolysis is carried out in an organic solvent such as ethers (tetrahydrofuran, dimethoxyethane, dioxane and the like), alcohols (methanol, ethanol, isopropanol, methoxyethanol and the like), ketones (methyl ethyl ketone and the like), amides etc. Alcohols, such as ethanol, are the preferred solvents. The hydrolysis is carried out in the presence of water and, if necessary, a catalyst. The amount of water to be used depends on the hydrolysis rate of the particular metal alkoxide and hydrocarbyl metal alkoxide, and the volume ratio of water to organic solvent can vary from e.g. 1:99 to 75:25. A catalyst may be necessary if hydrolysis in neutral water is too slow. An acid or a base can be used as a catalyst. For titania and zirkonia sol preparation, a catalyst may not be necessary. For silica sol preparation the conditions as described by De Lange et al. (J. Membr. Sci. 99 (1995), 57-75) can be followed. The hydrolysis temperature can be between ambient temperature and the boiling temperature of the organic solvent. It is preferred to use elevated temperatures, in particular above 40xc2x0 C. up to about 5xc2x0 C. below the boiling point of the solvent, e.g. up to 80xc2x0 C. in the case of ethanol.
The drying and/or calcination of the precipitate is preferably carried out under an inert, i.e. non-oxidising atmosphere, for example under argon or nitrogen. The calcination temperature is at least 100xc2x0 C., up to about 800xc2x0 C., preferably between 300 and 600xc2x0 C., using a commonly applied heating and cooling program. The porosity of the membranes can be tuned by selecting the specific metal (hydr)oxide precursor, the appropriate hydrolysis conditions, and the appropriate consolidation parameters (drying velocity, rate and temperature of calcination). Higher temperatures typically result in smaller pore sizes.
Applications of the supported microporous hydrophobic membrane of the invention are especially advantageous in humid process streams, where several problems are encountered by the moisture such as pore blocking and deterioration (e.g. hydrothermal degradation). Furthermore, hydrophobic species can be separated from hydrophobic and hydrophilic ones in liquid separation and gas separation, and especially in pervaporation. Isomer mixtures, such as butene/isobutene and p-/m-/o-xylene can also be efficiently separated into the individual components. Other useful applications are in air cleaning processes for the removal of dust particles or volatile organic compounds without the membrane being blocked by water.